This invention relates to polymer electrolyte fuel cells, and, more particularly, to polymer electrolyte fuel cells suitable for operation with hydrogen reformate from a gas or liquid fuel supply.
Practical fuel cells based on perfluorinated ionomer membranes (e.g., Nafion(trademark)) use reformed fuel as a primary source for the anode feed material. The reformate, besides hydrogen, may contain trace amounts of carbon monoxide (CO, from a few to hundreds ppm), whose presence is detrimental to the cell performance. Energy conversion in fuel cells typically depends on highly dispersed carbon-supported Pt, which catalyzes hydrogen electro-oxidation. However, CO strongly adsorbs on the Pt surface leading to a decrease of the Pt active surface area available for hydrogen oxidation, and, consequently, to losses in electrical current that are unacceptable for a practical device.
There have been various approaches attempting to achieve full CO tolerance in fuel cell performance. Full tolerance is typically defined as voltage losses no greater than 5% at any cell current in the presence of CO relative to that in its absence. For instance, binary or ternary Pt alloy-based catalysts have been tested. The alloy metals act as promoters of CO electro-oxidation and stripping. However, not even the best known of these catalysts, such as Ptxe2x80x94Ru alloys, are able to totally eliminate the detrimental effects of higher trace CO concentrations ( greater than 100 ppm) at 80xc2x0 C.
A second direction has been to increase the cell operating temperature above 100xc2x0 C. In polymer electrolyte fuel cells (PEFCs), this approach has been successful only for relatively short periods of time. At 120xc2x0 C., the sticking of CO onto Pt decreases enough to maintain the hydrogen electro-oxidation rate without significant losses. But higher operating temperatures bring additional problems to ordinary PEFCs, such as catalyst layer instability and ionomer membrane dehydration. These effects cause relatively rapid deterioration of the cell performance.
The best approach known so far is to bleed a small amount of air into the anode along with the fuel stream as described in U.S. Pat. No. 4,910,099, issued May 20, 1990, to Gottesfeld, incorporated herein by reference. Oxygen from the air is able to oxidize the CO adsorbed on the catalyst layer to CO2, which is released from the catalyst. The air cleans Pt sites, making them available for H2 electro-oxidation at an acceptable rate. Nevertheless, there are limits on how much air can be permitted into the cell without sacrificing fuel efficiency since oxygen in the air combines with the hydrogen fuel gas. Also, safety becomes an issue because of the potential explosive hazard presented by H2/O2 mixtures with increasing amounts of O2. These considerations indicate that injecting the minimum effective amount of air for a given content of CO is the most desirable condition for this approach.
U.S. patent application Ser. No. 09/216,313, filed Dec. 18, 1998, now abandoned, incorporated herein by reference, describes a new anode configuration that makes the air bleeding considerably more efficient in reaching CO-tolerance to levels of the order of 100 ppm, and with the concomitant advantage of lowering the total anode precious metal catalyst loading. The precious metal- based anode catalyst was distributed in two separate sections in the fuel cell. One fraction was pressed onto the polymer electrolyte membrane and the rest was placed on one side of the anode backing carbon cloth facing away from the membrane. The function of the catalyst on the backing was to allow the chemical oxidation of a CO impurity by O2 from the air bleeding at a distance from the electrocatalytic reaction. In this way, poisoning was avoided because CO was eliminated from the H2 fuel stream before H2 reaches the electrochemical catalyst layer where H2 electrochemical oxidation, the power generating process, takes place.
The present invention addresses the problem of reducing the level of CO in reformate fuel gas to acceptable levels using low cost, readily available non-precious metals. Novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
This invention may comprise a polymer electrolyte fuel cell (PEFC) usable in a reformate fuel stream containing diluted hydrogen fuel with CO as an impurity and with added air. A polymer electrolyte membrane has an electrocatalytic surface formed from an electrocatalyst mixed with the polymer and bonded on the anode side of the membrane. An anode backing is formed of a porous electrically conductive material and has a first surface abutting the electrocatalytic surface and a second surface facing away from the membrane. The second surface has an oxidation catalyst layer effective to catalyze the oxidation of CO by oxygen present in the fuel stream where the layer of oxidation catalyst is formed of a non-precious metal oxidation catalyst selected from the group consisting of Cu, Co, Fe, Tb, W, Mo, Sn, and oxides thereof.